1. Field of the Invention
The invention relates to a surge preventing circuit and, more particularly, to a surge preventing circuit, a local area network (LAN) connector using the same, and a network module using the same.
2. Description of the Related Art
A generation of a surge will affect a product, such as a computer or a mobile phone and so on, through a power transmission circuit or a ground route, and the affection of the surge may vary with an inrush route or inrush energy. When the surge directly rushes into the interior of the product, the damage to the product is the greatest. A great surge is generated from lightning strike, and a lightning rod is used for actively guiding the energy of the lightning strike in discharging. Electric current generated from the lightning strike may cause a sudden rise of the voltage at a ground resistor, and the electric current may be transmitted to the product through a coupling circuit embedded in the ground, result in damage of the inner elements of the product or failure of certain functions of the product. Therefore, a surge preventing device is a necessary device in a circuit.
According to a conventional surge preventing device, a transformer is disposed in a product circuit, and the transformer is used for reducing high voltage. The other method, a surge absorbing element such as a Zener diode or a metal oxide varistor (MOV) is used. A Zener diode is taken for example. Under a normal circuit voltage, the surge absorbing element does not work. When a surge is generated to cause a sudden rise of the voltage, since the external voltage of the surge absorbing element exceeds a breakdown voltage thereof to form a conductive circuit, at that moment, part of the current generated by a sudden rise of the voltage may be absorbed by the surge absorbing element, and the other part may be transmitted to a ground terminal through the surge absorbing element. Thus, a back-end circuit can be prevented from being damaged by the high current.
FIG. 1 is a circuit diagram showing a local area network (LAN) module having a surge preventing circuit in the prior art. Please refer to FIG. 1. A conventional local area network module 100 has a local area network connector 102 and a surge preventing circuit 104. The surge preventing circuit 104 includes three surge absorbing elements 104a, 104b, 104c and two resistors R1, R2. One side of the local area network connector 102 has a transmitting terminal TX, a center tapped transmitting terminal TXCT, a receiving terminal RX, and a center tapped receiving terminal RXCT. The transmitting terminal TX and the receiving terminal RX are coupled to a network (not shown), respectively. The center tapped transmitting terminal TXCT is coupled to a second terminal 2 of the surge absorbing element 104b, a first terminal 1 of the resistor R2, and a first terminal 1 of the surge absorbing element 104c. The center tapped receiving terminal RXCT is coupled to a first terminal 1 of the surge absorbing element 104a, a first terminal 1 of the resistor R1, and a first terminal 1 of the surge absorbing element 104b. In addition, the second terminals 2 of the resistors R1, R2 and the surge absorbing elements 104a, 104c are coupled to ground. The resistors R1, R2 can be coupled to ground though a capacitor C1.
In FIG. 1, when a surge is generated, the voltage suddenly rises to cause a sudden rise of current in an element. When the suddenly risen current flowing through the center tapped receiving terminal RXCT flows to a node A, part of the current may be absorbed by the surge absorbing element 104b or may flow to the ground terminal through the surge absorbing element 104a. In the same way, when the suddenly risen current flowing through the center tapped transmitting terminal TXCT flows to a node B, part of the current may also be absorbed by the surge absorbing element 104b or may flow to the ground terminal through the surge absorbing element 104c. Through the surge absorbing elements 104a, 104b, 104c, the current generated by the sudden rise of the voltage can be guided to the ground, thereby being capable of preventing the back-end circuit from being damaged by the high current.
However, although the surge absorbing element is used for protecting the circuit, the cost is high, and the number of the surge absorbing element increases with the number of the transformer.